In anesthesia or in intensive care, the condition of a patient is often monitored e.g. by analyzing the air exhaled by the patient for its carbon dioxide content. Therefore, a small portion of the exhalation air is delivered to a gas analyzer. This sample often carries along to the analyzer some water vapor, which condensates into droplets, and also some dust, mucus and blood. Such components carried along with the sample have a detrimental effect on the gas analyzer and measuring result. This is why the liquid components are often removed and collected from a gas sample upstream of the actual gas analyzer.
In prior known gas analyzers, e.g. U.S. Pat. Nos. 4,304,578 and 4,382,806, water has been removed from a gas sample by using a water separator, provided with a water-separation chamber, which divides the flow into two partial flows in a manner that the main flow is sucked through a measuring sensor by means of a tube connected with the water-separation chamber and a many times smaller side flow is sucked continuously by way of a tube connected with the bottom section of said water-separation chamber into a water receiver for retaining therein the water contained in a gas sample and further on to a pump. However, this solution is not totally sufficient, since some of the liquid components may still find access to the measuring sensor along with the gas sample. The response time of the gas analyzer may also increase because of the internal volume of the water-separation chamber.
It has also been known in the art, e.g. in U.S. Pat. No. 4,509,359, to use a moisture equalizing tube. In this case the analyzer is not usually fitted with an individual water separator but, instead, a sampling tube between a patient and the gas analyzer as well as a tube between a sampling connector in the analyzer and a measuring sensor are made of a material which equalizes moisture of the gas inside the tube to be the same as that on the outside, so that water always tends to find its way towards the drier side, the moisture of the gas sample equalizing to be the same as the moisture of ambient air and no condensation occurs on the tube walls.
This prior art solution has a fast response time but involves some serious drawbacks. The tube material is only capable of a limited transfer of water through the wall per unit time, whereby the water splashed from the tubing of a respirator, a patient's mucus or blood may end up in the measuring sensor. Dust in the air also finds its way to a measuring sensor and causes problems there.
Another improved fluid filtering device is described in U.S. Pat. No. 5,657,750. The upstream end of the sampling tube is provided with a tubular housing containing a hydrophobic hollow fiber filter element. In order not to increase the response time of the gas analyzer the tubular housing must have small volume. It is possible that the device can handle a small amount of water but it is easily obstructed by mucus or blood. The device would then have to be replaced. This may happen quite often in critical care use and would decrease the cost-effectiveness of the device.
In order to overcome the problems described above a special type of water separator was developed and the basic solution is described in U.S. Pat. No. 4,886,528. A passage, wherein a liquid component is separated from a gas flow, is divided into two sections by means of a gas permeable and liquid impermeable material. Thus a sample picked up from the exhalation air of a patient is delivered into the first passage of a water separator, from which the liquid component along with a minor amount of gas is sucked away, usually by way of a water receiver. Most of the gas flow received in the first passage is sucked through the liquid impermeable material into the second passage and further to a gas analyzer. This hydrophobic filter material prevents effectively the passage of liquid to the gas analyzer. In order to reduce flow resistance caused by the liquid impermeable material a certain contact area is necessary. To try to avoid an excessive increase in the response time of the gas analyzer the favored passages are kept narrow and elongated. The maximum cross-section area of a passage would preferably be close to that of the input conduit but in practice it is slightly larger for mechanical reasons. A larger input passage is e.g. less prone to clogging.
The last described solution works well as water separator but it has a major influence on the response time of the gas analyzer. In fact, its contribution to the response time is the most significant compared to the sampling line and the gas sensor with internal tubing. This is a drawback especially for an analyzer with low sample flow e.g. in neonatal gas measurement applications.